The booster dose of the COVID-19 vaccine is safe, according to a study published in The Lancet Respiratory Medicine that monitored heart measures through smartwatches.

Researchers at Tel Aviv University equipped close to 5,000 Israelis with smartwatches and monitored their physiological parameters over two years.

Of those monitored, 2,038 received the booster dose of the COVID-19 vaccine, allowing the researchers to objectively compare measures before and after the participants took the vaccine, and confirm the safety of the vaccine.

The team also examined the safety of the booster by analysing the medical files of 250,000 members of Maccabi Health Services anonymously.

They were able to evaluate the safety of the vaccines from three perspectives: subjectively -- what the participant reports, objectively -- what the watch detects, and clinically -- what the doctor diagnoses.

"The smartwatches were used to monitor a number of parameters such as heart rate, variation in heart activity, quality of sleep, number of daily steps taken, and more," said Professor Dan Yamin from Tel Aviv University.

"We saw clear and significant changes after administration of the vaccine, such as an increase in heart rate compared to the pulse rate measured before vaccination, and then we saw a return to the participant's baseline, i.e., the pulse levels after vaccination returned to their previous levels after six days. Hence, our study confirms the safety of the vaccine," Yamin said.

The researchers said the most surprising finding was that the watches were more sensitive than the people they were monitoring. Many participants reported fatigue, headache, etc. after receiving the vaccine, and after two or three days reported that they felt normal and well, they said.

"In contrast, from examining their watches, we saw distinct changes in heart rate that continued for several more days," Yamin said.

"There were also vaccinated participants who did not report any side effects at all and yet definitely experienced physiological changes, based on data from their smartwatches. In other words, we learned that the smartwatches were more sensitive to changes in general feeling than the participants themselves," he added.

In the medical literature, 25 unusual side effects attributed to the COVID vaccine were reported, and the researchers paid special attention to looking for rare cases of inflammation of the heart muscle (myocarditis) and pericarditis.

The researchers checked the frequency of these unusual side effects among a quarter of a million Maccabi members and found no increase in serious incidents of any kind associated with vaccination.